It was the first ceratopsian known to science, though relatively little is known about it. The original specimen consisted only of the animal's hip bones, hip vertebrae and ribs, and because these bones vary little between ceratopsid species, it is usually considered a nomen dubium. It is probably a synonym of Triceratops, but because the remains are so incomplete, they cannot be confidently referred to as the senior synonym of Triceratops.

The fossil remains of Agathaumas were first found in 1872 in southwestern Wyoming. They were discovered by Fielding Bradford Meek and H.M. Bannister while they were looking for fossil shells in the Lance Formation (then Laramie Formation) near the Black Butte and Bitter Creek. Meek and Bannister were employed by Ferdinand Hayden's Geological Survey of the Territories, and notified paleontologistEdward Drinker Cope of the find. Cope himself searched the ridge near Black Butte and re-discovered Meek's site, finding huge bones protruding from the rocks near a coal vein. The bones were preserved in sand and clay sediments, packed with fossil sticks and leaves, indicating a heavily forested habitat. Cope later (in 1873) described the skeleton as "the wreck of one of the princes among giants."Later in 1872, Cope published a description and name for the animal, Agathaumas sylvestris, or "marvelous forest-dweller," in reference to its great size and the environment revealed in the same rocks as its bones.[3] The name Agathaumas has been cited as an example of Cope's excitement with this discovery, which was, at the time, the largest known land animal that had ever lived (until several years later, with the discovery of the giant sauropod dinosaurs of the Morrison Formation.

Cope and his team eventually recovered complete hip bones, sacral vertebrae, and several ribs from the animal. Since these were the first ceratopsian remains found, Cope was uncertain as to precisely what sort of dinosaur Agathaumas was (and for a time considered it a hadrosaur) until O. C. Marsh described Triceratops in 1889.

In a 1889 paper, Cope suggested that Marsh's Ceratopsidae be renamed Agathaumidae, because of the paucity of Ceratops remains.

Agathaumas made It's 1st media appearance in the 1925 Dinosaur Film The Lost World where it fought & killed an Allosaurus . Then it encountered a Tyrannosaurus Rex. But then the T.Rex killed & ate the Agathaumas.